In a system in which an external apparatus, such as a portable music player or a digital camera, is connected to a host apparatus, such as a personal computer or a car navigation apparatus, the external apparatus may be supplied with electric power from the host apparatus via a connector. Normally, a power switch IC with a protection function is provided immediately upstream of the connector in order to protect a power supply of the host apparatus from any abnormality in the external apparatus. For example, in case the power supply line of the external apparatus connected to the host apparatus is short-circuited to ground (GND), the host apparatus limits the current supply to a constant current of 600 mA, using the power switch IC. At the same time, error information is sent to the host apparatus system. In this case, an average current that can be supplied to a normal external apparatus may be 500 mA at maximum in accordance with a standard specification.
FIG. 1 is a circuit diagram of a driver circuit and a current limiting circuit in a host apparatus according to related art. Specifically, the circuits include a driver circuit 201, an inverter circuit, and an operational amplifier 202. The driver circuit 201 includes a sense resistor and a N-ch (channel) power MOSFET QO. The operational amplifier 202 is connected to the gate of N-channel transistor Q1, and the drain of the N-channel transistor Q1 is connected to a charge pump circuit 50 that produces a voltage (such as 8 V) exceeding a power supply voltage.
FIG. 2 illustrates operation waveforms at various parts of the circuits of FIG. 1. VR1 is set to limit an output current when it exceeds 600 mA. When a load is connected that requires a current flow of 900 mA from an output OUT in a period between t1 and t4, an output current IOUT starts increasing at time t1 and a sense voltage VSENS decreases. When the output current IOUT reaches 600 mA at t2, the sense voltage VSENS decreases to VR1, at which the operational amplifier 202 is operated, so that an output VN2 of the operational amplifier 202 (at node N2) rises. While VG decreases when VN2 rises, the fall of VG is gradual because of a large transistor width and a large gate capacity of the power MOSFET Q0. In the period of decrease of VG, the output current IOUT of 900 mA flows, and the sense voltage VSENS is lower than VR1.
As VG further decreases, the output current IouT approaches 600 mA and the sense voltage VSENS approaches VR1 at which VN2 decreases. At t3, VG is constant at a voltage (=6V in the illustrated example of FIG. 2) such that the output current IOUT becomes constant (600 mA). At the same time, VN2 is constant (at 1V in the illustrated example) so that VG=6V. At t4, the output current IOUT becomes zero, when the sense voltage VSENS returns to 5V and VN2 reaches zero. Thereafter, VG rises slowly depending on the capacity of the charge pump 50.
In the waveform chart of FIG. 2, the time between t1 and t3 is a response time of the current limiting circuit. The response time may be on the order of 20 μs. The circuits illustrated in FIGS. 1 and 2 form an over-current protection circuit that supplies a load current of up to 500 mA. The over-current protection circuit is configured to lower VG in about 20 μs when a current limit value (such as 600 mA) is exceeded.
In some external apparatuses, large currents, such as 1 A, may flow during operation in a transient manner. When such an external apparatus is connected, the host apparatus may be required not to limit the current by the power switch IC even if the output current exceeds 500 mA as long as the excess is instantaneous. This is because the external apparatus that requires an instantaneous current flow of 1 A would not be able to operate normally if the load current is limited at 600 mA. Thus, the current limit value of the power switch IC may be set at a higher value, such as 1.2 A. However, in this case, if a current of 900 mA flows in the external apparatus due to abnormality, the host apparatus fails to limit the current and does not even recognize an error. As a result, the large current may keep flowing through the external apparatus, potentially causing the external apparatus to be overheated or even ignited.
JP Patent No. 3589392 discusses an over-current detection/protection circuit in which a current limit value is switched to a higher value for a period immediately after turning on a power MOSFET, where the current limiting value is brought back to a lower value when an inrush current has subsided. In this over-current detection/protection circuit, the over-current detection value is increased only immediately after the turning-on of the power MOSFET so that the inrush current can be allowed to flow. However, if a large current flows instantaneously due to an operation of the external apparatus after its operation current has stabilized, the large current is detected as an over-current. As a result, the output current is limited and the external apparatus fails to operate normally.